1. Field of the Invention
Example embodiments in general relate to a battery pack configured for powering cordless power tools and/or power tool systems including a tool and charger.
2. Description of Related Art
Over the past few years, lithium-ion (Li-ion) batteries have begun replacing nickel-cadmium (NiCd), nickel-metal-hydride (NiMH), and lead-acid batteries in low-voltage, portable electronic devices such as notebook-type personal computers. As compared to NiCd and NiMH batteries, Li-ion batteries are lighter but have a larger capacity per unit volume. For this reason, Li-ion batteries have been typically suitable to low-voltage devices that are preferably light and which are required to endure continuous use for a long time. In an over-discharged state, however, Li-ion batteries deteriorate rapidly, thus Li-ion batteries require over-discharge protection.
Conventional protections for Li-ion battery packs have been designed primarily for low-voltage portable electronic devices such as notebook-type personal computers, cellular phones etc., which require voltage generally on the order of 2 to 4 volts. Such devices are characterized by using battery packs composed of cells (such as Li-ion, NiCd, NiMH cells) that provide a maximum output voltage of about 4.2 volts/cell. For Li-ion battery cells, care must be taken to prevent damage from electrical and mechanical stresses, since lithium is a highly reactive substance.
However, much higher voltages (e.g., 18V or higher) are typically required for cordless power tools. However, above 18V tool systems, the overall weight of a cordless power tool using conventional NiCd or NiMH battery packs begins to become prohibitive. In other words, with NiCd and NiMH cordless power sources, higher power means substantially heavier battery packs. The corresponding increase in overall weight of the cordless tool makes the tool more difficult to manipulate and/or use over extended periods. For example, the weight of a 24 volt NiCd battery pack (about 3.3 lbs) represents over a 100 percent increase in weight as compared to the weight of a 12 volt NiCd battery pack (1.5 lbs).
Accordingly, battery packs based on a Li-ion or other lithium-based chemistries are being developed for cordless power tools in order to provide higher voltage outputs than conventional NiCd and NiMH battery packs (and substantially higher power than conventional Li-ion packs used for PCs and cell phones), but at a reduced weight (as compared to conventional NiCd or NiMH battery packs used as power sources in conventional cordless power tools). A feature of these battery packs is that these packs may exhibit substantially lower impedance characteristics as compared to conventional NiCd, NiMH and/or even the lower power Li-ion battery packs.
The introduction of lower impedance chemistries and construction styles to develop secondary batteries generating substantially higher output voltages (of at least 18 V and up, for example) may possibly create several additional protection issues. Battery packs having lower impedance also means that the pack can supply substantially higher current to an attached electronic component, such as a power tool. As current through a motor of the attached power tool increases, demagnetization forces (e.g., the number of armature turns of the motor times the current, ampere-turns) could substantially increase beyond a desired or design limit in the motor. Such undesirable demagnetization could thus potentially damage the motor.
For example, a lower impedance electrical source could cause damage to a tool's motor when the tool is held at stall condition. During motor stall, the motor and battery impedances are the only mechanisms to limit the current since there is no back-EMF created by the motor. With a lower impedance pack, the currents would be higher. Higher currents through the motor will increase the likelihood of de-magnetization of the permanent magnets within the tool's motor, possible thermal overload, eventual pack and/or tool damage, etc.
Accordingly, additional protection controls may need to be in place to address potential fault conditions (such as over-charge, over-discharge, over-current, over-temperature, cell imbalance, etc.) that could occur in high power Li-ion battery packs adapted for use with cordless power tools and chargers, so as to prevent internal or external damage to the pack, and/or to a connected charger, tool or user in the vicinity of a battery pack connected to a charger or tool.